The present invention relates in general to monitoring the proper operation of fluid control valves, and, more specifically, to an integrated monitor for sensing both proper movement of the valve element and the absence of pressure in the valve outlet with the valve element deactuated.
Fluid valves are important components of modem industrial control and manufacturing systems. For example, they are used in controlling the application of pressurized air to pneumatically-operated machines such as presses and other machine tools. It is often desirable or necessary to monitor the position of automatically controlled valves to ensure that a particular valve properly actuates and deactuates appropriately. Monitoring may also be necessary to ensure the safety of the human operators. A monitoring signal may be used to generate a visual or audible indication of a malfunctioning valve, may be used to automatically deactivate system operation in response to a fault, or both.
Many different types of sensing technologies have been used for monitoring valve position. One such technology is a magnetic sensor. For example, a movable valve element is configured to affect a magnetic field at a predetermined sensing location as the valve element moves between an actuated and a deactuated position. The magnetic field can be generated by a permanent magnet (either moving or stationary) or by an electromagnet. The magnetic sensor detects the magnetic field and generates a signal. This signal is sent to additional circuitry that provides monitoring information. A magnetic sensor has the advantage of having minimal interference with operation of the movable valve element, but has the disadvantage of being relatively expensive compared to other sensing technologies.
Pressure switches have also been used to monitor valve performance. For example, assuming a source of pressurized fluid (e.g., air) is present, the position of a valve element can be monitored by detecting the presence of pressurized fluid in the outlet of the valve. Pressure switches can be accommodated with essentially any kind of valve without affecting the design of the moving elements; however, they are also relatively expensive.
One of the more cost-effective sensing technologies is an electric sensing switch. An electric sensing switch is mechanically connected to a valve so that the conduction state of the switch is determined according to the position of the valve element. Electric sensing switches, however, have been able to sense only the movement (i.e., position) of the valve internal element to which it is connected. These switches are not able to sense all possible valve failures, such as a leaking poppet, that allow pressurized fluid at the valve inlet to reach the valve outlet even though the valve internal element has properly moved to its deactuated position. Therefore, a separate pressure switch has been needed in order to monitor fully the state of a valve.
The ability to sense a failed valve state when pressurized fluid is present at the outlet port even though the valve is shut off can be critical to safe use of fluid valves. In a 3-port valve application (i.e., a valve with inlet, outlet, and exhaust ports), a leaking valve poppet might not result in significant pressure at the outlet port because of the pressure relief provided by the exhaust port. If, however, a silencer or filter attached to the exhaust port becomes backed up (i.e., clogged) then an undesirable pressure can build up in the outlet port. Thus, in safety critical applications, such as controlling pneumatic presses using a 3-port valve, it may be desirable to sense a leaky poppet quickly. In a 2-port valve application (i.e., a valve with an inlet and an outlet port), an exhaust port is not present. Thus, a failure will result in high pressure being present at the outlet port, which is undesirable. The use of 2-port valves may be advantageous in applications that use a fluid other than air (e.g., nitrogen) to prevent the fluid from escaping to the atmosphere due to the cost of the fluid and contamination concerns. Hence, it is desirable to detect both failure states of the valve as discussed above.